During recent years it has become evident that retinoids (vitamin A-derivatives) have a variety of biological functions. Most of the interest concerning retinoids has been to focus on the role of retinoic acid (RA) during embryogenesis and in particular on the role of RA in pattern formation of the vertebrate limb (Tabin, Cell 66:199-217 (1991)). However, it is well established that retinoids play important roles in other normal physiological functions; for example retinoids have been shown to be important for normal differentiation and growth of several epithelia including skin (Fuchs, J. Cell Biol. 111:2807-2814 (1990)). The recent identification of nuclear RA-receptors (Benbrook et al., Nature 333: 669-672 (1988); Brand et al., Nature 332: 850-853 (1988); Giguere et al., Nature 330:624-629 (1987); Krust et al., PNAS 86: 5310-5314 (1989); Mangelsdorf et al., Nature 345: 224-229 (1990); Petkovich et al., Nature 330:444-450 (1987); Zelent et al., Nature 339: 714-717 (1989)) have demonstrated that the nonvisual function of retinoids, i.e., RA, is to control transcription of specific genes and a number of such genes have been identified (de The et al., Nature 343:177-180 (1990); Duester et al., Mol. & Cell Biol. 11: 1638-1646 (1991); La Rosa et al., PNAS 85:329-333 (1988); La Rosa et al., Mol. & Cell Biol. 8: 3906-3917 (1988); Nicholson et al., EMBO J 9:4433-4454 (190); Vasios et al., PNAS 86:9099-9103 (1989)).
The mechanisms involved in controlling the level of RA available to the nuclear RA-receptors are less well known but it is known that under normal physiological conditions most cells obtain retinoids as retinol. The extracellular transport of retinol is carried out by the plasma retinol-binding protein (RBP) (Goodman, in Sporn et al., Eds. The Retinols: 41-88 (Academic Press, 1984)). This 21 KDa protein is well characterized and both the primary and tertiary structures are known (Newcomer et al., EMBO J 3: 1451-1454 (1984); Rask et al., FEBS Letters 104: 55-58 (1980)). RBP is structurally related to a number of extracellular proteins involved in the transport of small hydrophobic compounds, called the lipocalins (Pervaiz et al., FASEB J 1:209-214 (1987)). Well known members of the lipocalin group of protein include .beta.-lactoglobulin (Godovach-Zimmerman et al., Hoppe-Seyler Biological Chemistry 366:431-434 (1985)) apolipoprotein D (Drayna et al., J. Biol. Chem. 261: 16535-16539 (1986)), olfactory binding protein (Lee et al., Science 235:1053-1056 (1987)) and protein HC (Lopez et al., Biochem. & Biophys. Res. Comm. 103:919-925 (1981)).
The details of how retinol is transferred from RBP to cells are not known. However, the specific transfer to a restricted number of cells and cell types suggest a receptor mediated mechanism. The presence of RBP-receptors has been recognized on several cell types (B avik et al., J. Biol. Chem. 266:14978-14985 (1991); Eriksson et al., Canc. Res. 46:717-722 (1986); Heller, J. Biol. Chem. 250:3613-3619 (1975); McGuire et al., Endrocrin 108: 180-188 (1981); Rask et al., J. Biol. Chem. 251:6360-6366 (1976); Sivaprasadarao et al., J. Biochem. J. 255: 561-569 (1988)). Recently a membrane binding assay was developed by which some of the characteristics of a RBP membrane receptor expressed in bovine retinol pigment epithelium (RPE) were revealed (B avik et al., supra, Eriksson et al., supra). Some partial purification of this receptor is disclosed in U.S. Ser. No. 740,006 filed Aug. 2, 1991, now abandoned.
Monoclonal and polyclonal antibodies to proteins which are receptors for retinol binding protein have now been found useful in securing pure RBPr from RPE cells. This molecule is characterized by a molecular weight of about 63 KDa, as determined by SDS-PAGE. In addition, complementary DNA ("cDNA") has been isolated which codes for these lipocalin receptors, especially the RBPr. This DNA has been incorporated into plasmids, and has been successfully translated in vitro. These, as well as other aspects of the invention are elaborated upon in the disclosure which follows.